Active implantable medical devices collect signals and deliver pacing pulses through electrodes that are integrated in a lead. The lead is connected to the device's power supply and circuitry through a connector.
The electrodes are intended to contact the tissue(s) to be stimulated, or tissue(s) from which an electrical signal is to be collected; for example, myocardium, nerve, muscle, etc. In the case of a device for cardiac diagnosis and therapy, these electrodes can be endocardial electrodes (located in a cavity of myocardium and in contact with a wall thereof), epicardial electrodes (preferably, for defining a reference potential or for applying a shock), or intravascular electrodes (for example, the lead may be introduced through the coronary sinus to a location in front of the left ventricle wall).
Recent developments in the domain of so-called “multisite” devices has increased the number of electrodes that these devices can use and now permits single or multiple stimulation and sensing sites to improve the operation of these devices.
In the particular case of ventricular resynchronization devices (so-called “CRT” devices, for Cardiac Resynchronization Therapy), referred to here in a non-limiting manner, a device equipped with electrodes to pace both ventricles must be implanted in a patient. Pacing the right ventricle (and right atrium) is performed by a standard endocardial lead. However, it is very difficult to access the left ventricle. To pace the left ventricle, usually, a lead is introduced through a coronary vein in the epicardium and the tip of the lead must be positioned in front of the left ventricle. The implanting procedure is very difficult because the diameter of the coronary vessels decreases as the lead progresses and it is not easy to find the optimal position for the lead during the implantation procedure. Further, the proximity of the lead to the phrenic nerve may result in undesirable pacing.
So-called “multi-electrode” leads have been developed to palliate these difficulties. As an example, multi-electrode leads can be equipped with ten electrodes and the lead(s) providing the best pacing can be chosen after implantation. Electrode selection may be done automatically by measuring the endocardial acceleration peaks (EAP, also referred to as PEA), measuring bioimpedance, or based upon any other kind of sensor likely to provide information on the hemodynamic status of the patient. Electrode selection may also be done “by hand” by the practitioner, by means of a suitable intelligent programmer communicating with the pulse generator.
Because the lead body must have a very narrow diameter, it is not possible to embed as many conductors as electrodes within the lead because the lead would be unacceptably wide. The diameter of the lead is important for the lead itself and also so that the connector can be level with the pulse generator.
For these reasons, systems of electrode multiplexing have been developed to allow the multiplicity of electrodes to interface with two conductors connected to the pulse generator's terminals (the two terminals are hereinafter referred to as “distal” and “proximal”, similar to the location of corresponding electrodes of a simple endocardial bipolar lead). In a simplified embodiment, these two conductors may be replaced by a single conductor (corresponding to a simple unipolar endocardial lead), in which the pulse generator's case ensures the circuit feedback, through the patient's body tissues. The particular embodiment using a single conductor can be adapted to one in which the lead comprises two conductors.
United States Published Patent Application No. US 2006/0058588, issued as, U.S. Pat. No. 7,214,189 (filed Dec. 1, 2005) describes a device in which the pulse generator is connected to a multi-electrode lead by two conductors via a multiplexer/demultiplexer circuit. These two conductors collect depolarization signals and deliver pacing pulses. The two conductors also deliver logic signals to the multiplexer/demultiplexer which provide control over the selection switches for one or more lead electrodes. These signals also deliver energy that is required for the operation of the multiplexer/demultiplexer circuit and switches.
The multiplexer/demultiplexer circuits and switches are preferably located at the lead tip. In a preferred embodiment, the commuters are micro electromechanical systems (MEMS), technologically integrable into the substrate of a chip that can be embedded within the lead body. Such components are, for example, described in United States Published Patent Application No. US 2004/0220650, issued as, U.S. Pat. No. 7,474,923 (filed Apr. 29, 2003).
In the prior art techniques, notably those disclosed in United States Published Patent Application US 2003/0149456 (abandoned), the switches are controlled by sending a coded series of logic pulses to the circuit located at the lead tip. Each sequence of pulses determines one of the switches to be set to “on” or “off” in a univalent way. However, in order for the circuit to detect that the collected signal is a coded sequence of pulses and not a pacing signal, the pulse sequence is initiated with a logic pulse sequence coding a byte to zero. Decoding of this byte allows the circuit to recognize that the signal that will follow is a command signal, and this circuit then opens all the switches so as to electrically isolate the electrodes from the pulses that will then be delivered. Once the pulse sequence has been collected and analyzed, the circuit actuates the switches according to the particular configuration corresponding to this sequence code.
This technique, however, presents a risk. When the byte defining the start of the coded pulse sequence is set to zero and applied to the circuit, the corresponding logic level is equally applied to at least one of the electrodes—the one(s) that is(are) commuted by the switches—because the switches at that time have not all been scheduled to open. This creates a risk that is not negligible for the patient because the logic levels of the pulse sequences have non-negligible levels of voltage and are likely to induce undue fibrillation, therefore creating a potentially fatal risk to the patient.
For these reasons, the technique proposed by the prior art is mainly restricted to the selection of one or many optimal electrodes at the time of implantation, but is not suited to any post implantation programming, either automatic or manual, of the configuration of commuted electrodes.